The successful implementation of a light wave communication system requires high quality light guide fibers having mechanical properties sufficient to withstand the stresses to which they are subjected. Each fiber must be capable of withstanding over its entire length a maximum stress level to which the fiber will be exposed during installation and service. The importance of fiber strength becomes apparent when one considers that a single fiber failure will result in the loss of several hundreds of circuits.
The failure of light guide fibers in tension is commonly associated with surface flaws which cause stress concentrations and lower the tensile strength below that of pristine unflawed glass. The size of the flaw determines the level of stress concentration and, hence, the failure stress. Even micron-sized surface flaws cause stress concentrations which significantly reduce the tensile strength of the fibers.
Optical fibers are normally made in a continuous process which involves drawing a thin glass strand of fiber from a partially molten glass preform and thereafter applying the coating layers. A furnace is used to partially melt the preform to permit the fiber to be drawn. The heat of the furnace and the rate of draw of the fiber must be in proper balance so that the optical fiber can be drawn continuously under uniform conditions. Long lengths of light guide fibers have considerable potential strength, but the strength is diminished by airlines or holes occurring in the optical fibers. Furthermore, airlines in optical fibers also interfere with the light-propagation properties of the optical fibers.
Soon after an optical fiber is drawn, the optical fiber is coated with a layer of a coating material such as, for example, a polymer. This coating serves to prevent airborne particles from impinging upon and adhering to the surface of the drawn fiber, which would weaken it or even affect its transmission properties. Also, the coating shields the fibers from surface abrasion, which could occur as a result of subsequent manufacturing processes and handling during installation. The coating also provides protection from corrosive environments and spaces the fibers in cable structures. The above-referenced co-pending related applications, Ser. Nos. 08/815,180 and 08/814,673, which are incorporated herein by reference, are directed to detecting defects in an optical fiber coating and detecting and distinguishing between defects in an optical fiber coating, respectively.
It is generally known in the industry to monitor optical fibers as they are being drawn during the manufacturing process to determine whether defects exist in the optical fibers. However, the known techniques analyze the optical fibers during the drawing process before the coating layers have been applied and do not analyze the outer surface of the coated fiber to detect characteristics or qualities relating to the outer surface of the outer coating.
For example, Bondybey et al., U.S. Pat. No. 4,021,217, disclose a system for detecting optical fiber defects to determine the tensile strength of optical fibers as they are being manufactured prior to any coating layers being applied to the optical fiber. The apparatus disclosed in the Bondybey et al. patent projects a focused beam of monochromatic light onto an optical fiber as it is being drawn. A photodetector, such as a photomultiplier, is positioned off axis with respect to the direction in which the light is projected onto the optical fiber so that it receives only scattered light unique to defects contained in the fiber. The output of the detector is received by an electrometer strip chart recorder which plots a scattering trace corresponding to the light detected. The peaks in the scattering trace correspond to defects in the optical fiber.
Button et al., U.S. Pat. No. 5,185,636, disclose a method for detecting defects such as holes in a fiber. The apparatus disclosed in the Button et al patent utilizes a laser for projecting a beam of light onto the optical fiber. Two optical detectors are positioned on each side of the optical fiber. As a result of the coherence and monochromaticity of the laser beam, interference patterns are created in the far field which are detected by the optical detectors. Holes contained in the optical fiber result in fewer fringes in the interference patterns created in the far field. A plurality of light sources are used in order to ensure that light passes through the entire fiber so that no blind spots exist. This is intended to ensure that light will be reflected off of holes contained at any location within the optical fiber and thus will be detected by the optical detectors. Spatial frequency spectra are generated based on the output of the light detectors and the spectra are analyzed to determine whether a hole exists in the optical fiber.
The systems disclosed in Button et al. and Bondybey et al. both perform optical detection of defects in an optical fiber before any coating layers have been applied to the optical fiber. Therefore, these systems do not detect surface characteristics or qualities in the outer surface of a coated optical fiber. The above-referenced co-pending related application having Ser. No. 09/015,460, which is incorporated herein by reference, is directed to detecting defects inside of the optical fiber itself.
In the optical fiber industry, it is common to apply a layer of ink to the outer coating layer of the optical fiber cable. Different color inks are applied to different optical fibers to allow a technician to distinguish between different optical fibers, such as, for example, a transmitting optical fiber and a receiving optical fiber. A well known industry standard defines the colors that are used for different optical fibers in order to distinguish between them However, the coated optical fiber is typically covered with a strength member and a portion of the strength member must be removed in order to ascertain the color of the ink applied to the outer coating layer. Normally, the technician looks at the end of the optical fiber cable to determine the color of the ink applied to the outer coating layer. The technician may be required to remove a portion of the strength member to ascertain the color of the ink applied to the outer coating layer.
A problem sometimes encountered by the technician is that a break in the ink applied to the outer coating layer has occurred, thus making it difficult or impossible for the technician to ascertain the color of the ink. When this happens, the technician may be unable to distinguish between different optical fibers. It is also known that breaks in the ink, sometimes referred to as ink skips, can produce added optical loss. This can occur if the spatial frequency of a skip is located in a critical region for micro-bending.
Accordingly, a need exists for a system for detecting qualities or characteristics in the outer surface of coated optical fibers, such as, for example, a break or inconsistency in the ink layer applied to the outer coating layer of the optical fiber.